1. Field of the Invention
The present invention relates to a resolver for use in measuring a rotational speed of a motor.
2. Description of Related Art
The resolver has a similar structure to that of the motor and includes a rotor and a stator. The stator core is provided with a plurality of magnetic pole teeth like the stator core of the motor, and a plurality of winding wires are wound around the plurality of the magnetic pole teeth in succession like the stator core of the motor while the core is insulated with a resin mold. In a typical resolver, each magnetic pole tooth is wound with three winding wires. One of them is for excitation and the other two wires are for outputting sine waves and cosine waves.
Both end portions of the plurality of the winding wires are gathered at one place on the external circumference of the stator core, where a connector is formed. At the connector, a controller for the resolver is connected to end portions of the winding wires. The controller for the resolver supplies the winding wires for excitation with a voltage (current) of sine waves while measuring a voltage (current) of the winding wires for output so as to calculate the rotational speed of the rotor. In the meantime, both end portions of the winding wires are referred to as lead wire in this specification.
Usually, the connector is also manufactured of resin. Generally, the connector is formed integrally with a resin mold, which covers the magnetic pole teeth. The lead wires are wound on the resin which covers the magnetic pole teeth while they are fixed to the resin of the connector with varnish or the like. Examples of the resolvers having such a structure have been disclosed in Japanese Patent Application Publication No. 2003-028670 (JP 2003-028670 A) and Japanese Patent Application Publication No. 2003-207369 (JP 2003-207369 A).
By the way, resin has a relatively high linear expansion coefficient (thermal expansion coefficient). Thus, there is a fear that the lead wires which cover the magnetic pole teeth or the lead wires fixed to the resin of the connector may be broken due to thermal expansion of the resin. According to the technology of JP 2003-028670 A, in order to prevent the lead wires from being broken, a coil wound around the magnetic pole tooth is molded with first resin while winding wires (jumper wires) stretched between adjacent magnetic pole teeth are molded with second resin. As the second resin, a material having a lower hardness than the first resin is selected. This technology prevents the jumper wires from being broken by applying the material having a lower hardness to the second resin for covering the jumper wire.